1. Field of Invention
The present invention relates to the field of tropospheric scatter radio links and more particularly to a radiating system with angular diversity comprising an antenna reflector, at least a first and a second antenna horn, and waveguides connected with said antenna horns.
2. Description of the Prior Art
It is known that to establish microwave radio links beyond the horizon it is possible to use radiating systems which utilize the scattering of electromagnetic waves by the troposphere.
It is also known that the troposphere displays irregularities generally considered as bubbles or layers which vary continuously in number, form and position with resulting variation of the refraction index and diffusion angle. When such irregularities are illuminated by a beam of electromagnetic waves from a transmitting antenna they scatter the electromagnetic energy in all directions but predominantly within a cone having as its axis the direction of transmission.
It is clear that with such link path attenuation is much higher than that found in links with antennas which remain in a field of mutual visibility since the propagation mechanism is different. In addition, in troposcatter radio links there are met sudden deep fadings of the intensity of the signal received due mainly to random movements of the troposphere layers.
Diversity techniques are known which are used to avoid the aforementioned problems with tropospheric propagation, i.e. spatial, frequency, polarization and angular diversity, for the purpose of increasing the reliability of the link.
Spatial diversity consists of transmitting the same signal with two antennas appropriately spaced and directed and in using two other antennas similarly arranged for reception. The basic assumption on which this technique is based is that fadings of signal intensity which appear on the two beams are poorly correlated.
Frequency diversity differs from spatial diversity in that the signal is radiated on a single beam but with two carriers appropriately spaced in frequency so as to decorrelate intensity fadings of the two signals received.
Polarization diversity consists of radiating the signal on a single beam with two polarizations orthogonal to each other (generally horizontal and vertical) and at the same frequency in such a manner as to decorrelate the fadings of the two signals received.
Angular diversity consists of radiating electromagnetic power in a single beam and in equipping the receiving antenna with two receiving horns appropriately spaced from each other in such a manner that the single transmitted beam is received in two different directions forming a certain angle called diversity angle and giving rise to two signals as independent as possible from the point of view of tropospheric propagation. It is thus possible to effect in reception a combination of the two signals received, such that the combination signal intensity or the signal-to-noise ratio of the combination is always kept sufficiently high.
Combinations of the aforementioned diversity techniques such as, for example, space-frequency and space-polarization, etc . . . diversity are also possible and commonly accomplished.
It is also known that with angular diversity systems there is the problem of optimizing the diversity angle which, as aforementioned, depends on the distance between the receiving horns. As the diversity angle increases so does the statistical independence between the intensity fadings which appear on the two received signals, with a resulting system improvement. But antenna gain is simultaneously reduced because of defocusing. In addition the transmissive characteristics of the troposphere vary dpending on the different climatic zones of the earth so that an optimized diversity angle for a given place is inapplicable in another. These drawbacks become even more serious for mobile antennas which are moved from one place to another. Frequently and for which the optimal diversity angle is consequently nearly never obtained.
An angular diversity radiating system is described in the article of Sigheru Morita, Hiroki Tachibana, Toshinari Hoshino and Hitoshi Kawasaki entitled "Effect of Angle Diversity in Troposcatter Communication System" published in Nec Research & Development, No. 45, pages 83-93, April 1977.
The Morita, et al system accomplishes angular diversity by means of two double-polarization horns both capable of transmitting and receiving or by means of two antenna horns of which the first, with double polarization, is used both to transmit and receive and the second, with single polarization, is used only for receiving.
The main drawbacks met with in the Morita, et al system are the consequence of the fact that it is not possible to optimize the diversity angle in relation to the site where the system is installed and of the fact that the horn apertures are rectangular, and therefore in double polarization only one polarization can be optimized.
Accordingly the object of the present invention is to overcome the drawbacks described hereinbefore and provide an angular-diversity radiating system which provide permits optimization of the diversity angle for the place where the system is installed.